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Application Report
SLVA680 – February 2015
ESD Protection Layout Guide
Guy Yater
............................................................................................................ High Volume Linear
ABSTRACT
Successfully protecting a system against electrostatic discharge (ESD) is largely dependent on the printed
circuit board (PCB) design. While selecting the proper transient voltage suppressor (TVS) founds the basis
of an ESD protection strategy, its scope is not covered here. ESD selection guides are available in
Technical Documents at www.ti.com/esd for guidance in choosing the correct type of TVS diode for a
particular system. With the proper TVS selected, designing a PCB Layout that leverages the strategies
outlined in this ESD Layout Guide will provide the PCB designer with an avenue towards successfully
protecting a system against ESD.
1
2
1
Contents
Introduction ...................................................................................................................
1.1
Optimizing Impedance for Dissipating ESD .....................................................................
1.2
Limiting EMI from ESD .............................................................................................
1.3
Routing with VIAs ...................................................................................................
1.4
Optimizing Ground Schemes for ESD ...........................................................................
Conclusion ....................................................................................................................
1
3
4
5
6
8
Introduction
An ESD event rapidly forces current (see Figure 1), IESD , into a system, usually through a user interface
such as a cable connection, or a human input device like a key on a keyboard. Protecting a system
against ESD using a TVS relies upon the TVS being able to shunt IESD to ground. Optimizing a PCB
Layout for ESD suppression is largely dependant on designing the path to ground for IESD with as little
impedance as possible. During an ESD event, the voltage presented to the protected integrated circuit
(Protected IC), VESD, is a function of IESD and the impedance presented to it. Since the designer has no
control over IESD, lowering the impedance to ground is the primary means available for minimizing VESD.
Lowering the impedance presents several challenges. Mainly, it cannot be of zero impedance, or the
signal line being protected would be shorted to ground. In order for the circuit to have a realistic
application, the protected line needs to be able to maintain some voltage, usually under a high
impedance to ground. This is where the TVS becomes applicable.
IPEAK
30
Current (A)
90% IPEAK
20
10
800 ps 90/10 rise time
10% IPEAK
0
0
20
40
60
Time (ns)
80
100
Figure 1. IEC 61000-4-2 Compliant Level 4 (8 kV ESD) Waveform
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Introduction
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A TVS is an array of diodes (see Figure 2 for a
typical example) arranged to present a very high
impedance to the voltages normally present in
the circuit, but if voltages exceed the design, the
TVS diodes will breakdown and shunt IESD to
ground before it can damage the system being
protected. The system designer is then
challenged to lower the impedance for IESD from
the ESD Source through the TVS to ground.
ESD Source
TVS
Protected
Line
Protected IC
Figure 2. A Typical ESD Protection Scheme
The impedance presented to IESD is a function of any impedance inherent with the TVS (in the diode
array and the package of the TVS) and the PCB Layout between the ESD Source and the TVS ground.
A TVS is generally designed to offer as low of an impedance to ground for IESD as its overall design
constraints will allow. With the proper TVS selected, a critical phase of the design is to lower the
impedance in the PCB Layout between the ESD Source and the TVS ground.
Another concern created by the rapidly changing IESD is its associated rapidly changing electromagnetic
field (EM) causing interference (EMI) to couple onto other circuits of the PCB. This is especially true in
the area between the ESD Source and the TVS. Once the TVS shunts IESD to ground, the trace between
the TVS and the Protected IC should be relatively free of EMI. Therefore, unprotected circuits should not
be adjacent to an ESD protected circuit's traces between the ESD Source and the TVS. In order to keep
EMI emissions at a minimum, circuit traces between the ESD Source and the TVS should have corners
which do not exceed 45° or, ideally, which are curved with large radii.
In today's PCB Layout, board space is at a premium. ICs, including TVSs, are designed to be very
compact. Also, the density of their placement on the PCB is continually increasing. Multiple layer PCB
boards and routing lean heavily upon VIAs for maximizing the density to increase the system's feature
set while decreasing the system's size. This PCB architecture, particularly related to layer switching and
VIAs, plays an important role in shunting IESD to ground through the TVS. Large differences in VESD at the
Protected IC can be induced by the manner in which the circuit is routed to the TVS using VIAs.
Generally, placing a VIA between the ESD Source and the TVS is detrimental, but in some
circumstances the designer is forced to do so. Even in these circumstance, if properly done, VESD can still
be minimized at the Protected IC.
Grounding schemes are critical in protecting against ESD. Having a chassis ground for the TVS that is
separated from the digital and/or analog ground by inductance provides very good protection against
ESD related failures. Yet it presents great challenges when routing high speed circuits across multiple
ground planes. For this reason, many designs use one common ground for the protected circuits. Ground
planes are necessary for the TVS to have success in dissipating IESD without increasing VESD. Electrical
connections to an earth grounded chassis, like a PCB grounded through-hole for a chassis screw,
immediately adjacent to the TVS ground and the ESD Source's ground (i.e. a connector shield) provide a
sound methodology in keeping ground shifts at Protected ICs to a minimum. If a system cannot utilize a
chassis earth ground, tightly coupled multiple layer ground planes can help keep ground shifts at
Protected ICs to a minimum.
To summarize these parameters, successfully protecting a system against ESD includes:
•
•
•
•
2
Controlling impedances around the TVS for dissipating ESD current, IESD
Limiting the effects of EMI on unprotected circuits
Properly using VIAs to maximize ESD dissipation by the TVS
Designing a grounding scheme which has very low impedance for the TVS
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Introduction
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PCB Layout Guidelines for Optimizing Dissipation of ESD
1.1
Optimizing Impedance for Dissipating ESD
Outside of controlled RLC values, PCBs have inherent parasitics which contribute to overall board
performance. Usually these parasitics are detrimental to the functionality of the design. An important
parasitic to consider when designing a circuit to dissipate ESD is inductance. Because (see Note 1,
below) VESD = Vbr_TVS + RDYN(TVS)IESD + L(dIESD/dt), and the term dIESD/dt is very large, the forced current in
an ESD event will cause large voltages to drop across any inductance. For example, in an 8 kV ESD
event as specified by IEC 61000-4-2, the dIESD/dt = (30 A)/(0.8×10-9 s) = 4 × 1010 A/s. So even with 0.25
nH of inductance an additional 10 V is presented to the system.
Note 1:
• Vbr_TVS is the voltage required for the
TVS to enter its breakdown region and
begin shunting IESD to ground.
• RDYN(TVS) refers to the resistance
through the TVS diode array while
operating in the breakdown region of
the IV curve.
ESD Source
L1
Protected
Line
L2
L4
L3
Protected IC
Figure 3. PCB Inductance around a Single-channel TVS
In Figure 3 four parasitic inductors are shown: L1 and L2 is the inductance in the circuit between the ESD
Source (typically a connector) and the TVS, L3 is the inductance between the TVS and ground, and L4 is
the inductance between the TVS and the Protected IC. Not considering VIAs, the inductors L1 and L4 are
generally dependant upon design constraints such as impedance controlled signal lines. However, IESD
can still be "steered" towards the TVS by making L4 much larger than L1. This is accomplished by placing
the TVS as near to the ESD Source as the PCB design rules allow while placing the Protected IC far
away from the TVS, for example near the middle of the PCB. This effectively creates L4 >> L1, helping
shunt the IESD to the TVS. Placing the TVS adjacent to the connector also mitigates EMI from radiating
into the system. The inductor shown at L2 should not be present in a well designed system. This
represents a stub between the TVS and the line being protected. This design practice should be avoided.
The Protected Line should run directly from the ESD Source to the protection Pin of the TVS, ideally with
no VIAs in the path. The inductor at L3 represents the inductance between the TVS and ground. This
value should be reduced as much as possible, and perhaps represents the most predominant parasitic
influencing VESD. The voltage presented to the node "Protected Line" will be VESD = Vbr_TVS + IESDRDYN(TVS) +
(L2 + L3)(dIESD/dt). Thus the PCB designer needs to minimize L3 and eliminate L2. Minimizing L3 is
covered in Section 1.4. Minimizing L1 is covered in Section 1.2 and Section 1.3.
Summary:
•
•
•
•
Minimize any inductance between the ESD Source and the path to ground through the TVS
Place the TVS as near to the connector as design rules allow
Place the Protected IC much further from the TVS than the TVS is to the connector
Do not use stubs between the TVS and the Protected Line, route directly from the ESD Source to
the TVS
• Minimizing inductance between the TVS and ground is critical
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1.2
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Limiting EMI from ESD
Fast transients like ESD with high di/dt can cause EMI without proper steps for suppression. For ESD,
the primary source of radiation will be in the circuit between the ESD Source and the TVS. For this
reason, the PCB designer should consider this region a Keep-Out area for unprotected PCB traces which
could damage the system by either having direct contact with an IC, or by carrying the EMI further into
the system where it could radiate more EMI. Even with no inductance at L1 (as shown in Figure 3) the
rapidly changing electric field during ESD can couple onto nearby circuits, resulting in undesired voltages
on unintended circuits. Having any induction at L1 amplifies the EMI.
Figure 4 shows an unprotected line
running adjacent to a protected line
between the ESD Source and the TVS.
This practice should be avoided. During
an ESD event there will be a large
dIESD/dt between the ESD Source and
the TVS. The traces on this path will
radiate EMI and any nearby traces could
have a current induced in them by the
EMI. If these traces have no TVS
protecting them, the induced current in
the unprotected line can cause system
damage.
If there are any VIAs on the protected
line between the ESD Source and the
TVS, these same principles apply to any
layer the VIA crosses, no unprotected
lines should be ran adjacent to the VIA.
ESD Source
Unprotected line
Potential EMI
Coupling
Protected
Line
Protected IC
Figure 4. EMI Coupling onto an Adjacent Unprotected
Trace
Another aspect of PCB Layout to consider is the style of the corners between the ESD Source and the
TVS. Corners tend to radiate EMI during IESD. The best method of routing from the ESD Source to the
TVS is using straight paths which are as short as possible. Beyond lowering the impedance in the path
to ground for IESD, shortening the length of this path also reduces the EMI being radiated inside the
system. If corners are necessary, they should be curved with the largest radii possible, with 45° corners
being the maximum angle if the PCB technology does not allow curved traces.
8 kV ESD Strike
8 kV ESD Strike
8 kV ESD Strike
Emmission
Emmission
Figure 5. Electric Field During an 8 kV ESD Event for Three Different Corner Types
In Figure 5 note that for a 90° corner, the corner is a strong source of EMI. The electric field at the corner
is at least 7 kV. This will lead to an electric arc (ionization) for any radius less than about 2.6 mm (in air).
The EMI for the 45° and curve are much less pronounced. To further show the effects of corner styles,
Figure 6 plots the crosstalk between parallel traces with these three corner types. The 90° corner has
much higher coupling than the others, especially in the ESD frequency content region.
4
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0
-10
-20
Magnitude (dB)
-30
-40
90°
-50
45°
-60
Curved
-70
ESD Frequency
content region
-80
-90
-100
10 MHz
1 MHz
100 MHz
1 GHz
10 GHz
Frequency [Log]
Figure 6. Inter-trace Crosstalk with 45°, 90°,and Curved Corners
Summary:
•
•
•
•
1.3
Do not route unprotected circuits in the area between the ESD Source and the TVS
Place the TVS as near to the connector as design rules allow
Route with straight traces between the ESD Source and the TVS if possible
If corners must be used, curves are preferred and a maximum of 45° is acceptable
Routing with VIAs
It is best to route traces on the PCB from the ESD Source to the TVS without switching layers by VIA.
Figure 7 shows two examples. In Case 1, there is no VIA between the ESD Source and the TVS, so that
IESD is forced to the TVS protection pin before the VIA in the path to the Protected IC. In this case the VIA
represents L4 shown in Figure 3. In Case 2, IESD branches between the Protected IC and the VIA to the
TVS protection pin. In this case the VIA represents L2 in Figure 3. This practice should be avoided. The
inductance of the VIA is between the TVS and the path from the ESD Source to the Protected IC. This
has two detrimental effects: Since current seeks the path to ground with the least impedance, the
Protected IC may take the brunt of the current in IESD and any current that does pass through the VIA will
increase the voltage presented to the Protected IC by LVIA(dIESD/dt).
Case 1: Best
Case 2: Worst
TVS
TVS
ESD Source
PCB
VIA
Protected IC
PCB
VIA
Protected IC
ESD Source
Figure 7. Routing with VIAs
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Case 3
TVS
There may be cases where the designer has no
choice but to place the TVS on a different layer
than the ESD Source. Figure 8 shows Case 3 ,
a variation to Case 2. In Case 3, IESD is forced to
the protection pin of the TVS before IESD has a
path to the Protected IC. This is an acceptable
compromise to Case 2.
VIAs
PCB
Protected IC
ESD Source
Figure 8. Routing with VIAs
These three cases represent examples when VIAs are used between the ESD Source and the Protected
IC. It is best to avoid this practice, but if necessary Case 1 is the preferred method, Case 2 should be
avoided, and Case 3 is acceptable if there is no alternative.
Summary:
• Avoid VIAs between the ESD Source and TVS if possible
• If a VIA is required between the ESD Source and the Protected IC, route directly from the ESD
Source to the TVS before using the VIA
1.4
Optimizing Ground Schemes for ESD
Successfully eliminating all the parasitic inductance between the ESD Source and the TVS will not be
effective without a very low impedance path to ground for the TVS. The TVS ground pin should connect
to a same layer ground plane that is coupled with another ground plane on an immediately adjacent
layer. These ground planes should be stitched together with VIAs, with one VIA immediately adjacent to
the ground pin of the TVS (see Figure 10).
Figure 9 shows the PCB Inductance around a single-channel TVS (as shown earlier in Figure 3). This
section considers only the inductance at L3. Recall that, with L2 eliminated, the voltage presented to the
Protected IC during an ESD event will be VESD = Vbr_TVS + IESDRDYN(TVS) + L3(dIESD/dt) and for 8 kV, dIESD/dt
= 4 × 1010. Clearly, L3 must be lowered as much as possible.
ESD Source
L1
Protected
Line
L2
L4
L3
Protected IC
Figure 9. PCB Inductance around a Single-channel TVS
6
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To lower L3, the TVS ground pin would ideally connect directly to a coupled ground plane. Figure 10
shows the ground pad of a TVS connected to the top layer ground plane. There are four stitching VIAs
connecting the top layer ground plane to an internal ground plane. These could connect more than one
ground plane layer depending on the layer count and board design. A grounded chassis screw is located
very near to the TVS ground pad as well. A grounding scheme resembling this yields a very low
impedance to ground for L3.
Chassis Screw
TVS Ground Pad
Ground Plane on Top Layer
Top Layer
Stitching
VIAs (x 4)
Mid Layer Ground Plane
Figure 10. Two Layer PCB - Top Ground Plane Stitched to a Mid-Layer Ground Plane
Figure 10 is not relevant for some types of TVSs due to package types. Those with BGA packages which
have the ground pin surrounded by other pins will need to VIA to an internal ground plane, preferably to
multiple, coupled ground planes. Figure 11 shows a TVS with such a ground pin.
GND Plane Detail
0.4 mm pitch BGA package
Copper pour
GND VIA:
0.254 mm (10 mil) pad,
0.152 mm (6 mil) drill.
Epoxy filled and plated.
Legend
Ground Pin
VIA in SMD
0.1 mm (4 mil) clearance VIA to copper
Figure 11. Grounding an Isolated Ground Pin in a BGA Package
VIAs need to be constructed to offer as little impedance as possible. Due to the "skin effect," maximizing
the surface area of the GND VIA minimizes the impedance of the path to ground. For this reason make
both the VIA pad diameter and the VIA drill diameter as large as possible, thus maximizing the surface
area of the outside of the VIA surface and the inside of the VIA surface. The ground plane should not be
broken in the vicinity of the GND VIA. If possible, attaching the GND VIA to a ground plane on multiple
layers minimizes the impedance. The GND VIA should be filled with a non-conductive filler (like epoxy)
as opposed to a conductive filler, in order to keep the surface area of the inside of the VIA created by the
drill. The GND VIA should be plated over at the SMD pad. Clearances between the GND VIA and nonground planes (i.e. power planes) should be kept at a minimum. This increases capacitance which
lowers impedance.
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Conclusion
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Conclusion
Designing ESD protection into a system can be successful with the proper techniques applied. Following
these ESD layout guide outlines will ensure the TVS has optimum conditions for dissipating the ESD.
In summary:
•
•
•
•
8
Control Impedances around the TVS for dissipating ESD current, IESD:
– Minimize any inductance between the ESD Source and the path to ground through the TVS
– Place the TVS as near to the connector as design rules allow
– Place the Protected IC much further from the TVS than the TVS is to the connector
– Do not use stubs between the TVS and the Protected Line, route directly from the ESD Source to
the TVS
– Minimizing inductance between the TVS and ground is critical
Limit the effects of EMI on unprotected circuits:
– Do not route unprotected circuits in the area between the ESD Source and the TVS
– Place the TVS as near to the connector as design rules allow
– Route with straight traces between the ESD Source and the TVS if possible
– If corners must be used curves are preferred and a maximum of 45° is acceptable
Properly use VIAs to maximize ESD dissipation by the TVS:
– Avoid VIAs between the ESD Source and the TVS if possible
– If a VIA is required between the ESD Source and the Protected IC, route directly from the ESD
Source to the TVS before using the VIA
Use a grounding scheme that has very low impedance:
– Connect the TVS Ground Pin directly to a same layer ground plane that has nearby VIAs stitching
to an adjacent internal ground plane
– Use multiple ground planes when possible
– Use a chassis screw, connected to PCB ground, near to the TVS and ESD Source (i.e. the
connector ground shield)
– Use VIAs of large diameter with a large drill, which lowers impedance
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